Thermosetting resins that are commonly used in fiber-reinforced composites cannot be reshaped after thermoforming. Errors in forming cannot be corrected, so these thermosetting resins are undesirable in many applications.
Although thermoplastic resins are well known, the use of fiber-reinforced thermoplastic resins is a relatively new art. Fiber toughens and stiffens the thermoplastic resin to produce high-performance composite products. A sheet of fiber-reinforced resin can be heated and then stamped into a desired shape with appropriate dies. The shape can be altered thereafter, if desired.
Thermoplastic resins commonly have a tendency to be weakened by organic solvents. Accordingly, circuit boards formed from conventional, fiber-reinforced thermoplastic resin composites usually cannot be cleaned with solvents that are commonly used in the aerospace industry. In structural aircraft applications, care must also be taken to eliminate contact between the composites and hydraulic or cleaning fluids. At moderate or high temperatures, many fiber-reinforced thermoplastic composites lose their abilities to carry load due to softening of the resin. Thus, improved thermal stability and solvent-resistance are desirable to fulfill the existing needs for advanced composites. The oligomers of the present invention provide such polyimide composites when they are cured.
Recently, chemists have sought to synthesize oligomers for high performance advanced composites suitable for aerospace applications. These composites should exhibit solvent resistance, toughness, impact resistance, ease of processing, and strength, and should be thermoplastic. Oligomers and composites that have thermooxidative stability and, accordingly, can be used at elevated temperatures are particularly desirable.
While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites. Accordingly, research has recently focused on polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness. Still the maximum temperatures for use of the polyimide composites, such as PMR-15, are about 600.degree.-625.degree. F., since they have glass transition temperatures of about 690.degree. F.
There has been a progression of polyimide sulfone compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode synthesized maleic capped linear polyarylimides as disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesized maleic or nadic capped imido-substituted polyester compositions as disclosed in U.S. Pat. No. 3,729,446. Monacelli proposed tetra-maleimides made through an amic acid mechanism with subsequent ring closure, as shown in U.S. Pat. Nos. 4,438,280 or 4,418,181. We synthesized thermally stable polysulfone oligomers as disclosed in U.S. Pat. Nos. 4,476,184 or 4,536,559, and have continued to make advances with polyetherimidesulfones, polybenzoxazolesulfones, polybutadienesulfones, and "star" or "star-burst" multidimensional oligomers. We have shown surprisingly high glass transition temperatures yet reasonable processing and desirable physical properties in many of these oligomers and their composites.
Polybenzoxazoles (or their corresponding heterocycles), such as those disclosed in our applications U.S. Ser. Nos. 07/116,592, now U.S. Pat. No. 4,965,356 (to Lubowitz & Sheppard) and 07/121,964, now U.S. Pat. No. 4,868,270 (to Lubowitz, Sheppard, and Stephenson), may be used at temperatures up to about 750.degree.-775.degree. F., since these composites have glass transition temperatures of about 840.degree. F. Some aerospace applications need composites which have even higher use temperatures while maintaining toughness, solvent resistance, ease of processing, formability, strength, and impact resistance.
Multidimensional oligomers, such as disclosed in our copending applications U.S. Ser. Nos. 07/000,605, now U.S. Pat. No. 5,210,213; 07/167,656; and 07/176,518, have superior processibility than some advanced oligomers since they can be handled at lower temperatures. Upon curing, however, the phenylimide and caps crosslink so that the thermal resistance and stiffness of the resulting composite is markedly increased. This increase is obtained with only a minor loss of matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties. Glass transition temperatures above 850.degree. F. are achievable.
Commercial polyesters, when combined with well-known reactive diluents, such as styfane, exhibits marginal thermal and oxidative resistance, and are useful only for aircraft or aerospace interiors. Polyarylesters are often unsatisfactory, also, since the resins often are semicrystalline which may make them insoluble in useable laminating solvents, intractable in fusion under typical processing conditions, and difficult and expensive to manufacture because of shrinking and/or warping. Those polyarylesters that are soluble in conventional laminating solvents remain so in composite form, thereby limiting their usefulness in structural composites. The high concentration of ester groups contributes to resin strength and tenacity, but also to make the resin susceptible to the damaging effects of water absorption. High moisture absorption by commercial polyesters can lead to lowering of the glass transition temperature leading to distortion of the composite when it is loaded at elevated temperature.
High performance, aerospace, polyester advanced composites, however, can be prepared using crosslinkable, end-capped polyester imide ether sulfone oligomers that have an acceptable combination of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. By including Schiff base (--CH.dbd.N--) linkages in the oligomer chain, the linear, advanced composites formed with polyester oligomers of our copending application U.S. Ser. No. 07/137,493, filed Dec. 23, 1987 can have semiconductive or conductive properties when appropriately doped or reacted with appropriate metal salts.
Conductive and semiconductive plastics have been extensively studies (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869; and 4,344,870), but these polymers do not possess the blend of properties which are essential for aerospace applications. That is, the conductive polymers do not possess the blend of (1) toughness, (2) stiffness, (3) ease of processing, (4) impact resistance (and other matrix stress transfer capabilities), (5) retention of properties over a broad range of temperatures, and (6) thermooxidative resistance that is desirable on aerospace advanced composites. The prior art composites are often too brittle.
Thermally stable multidimensional oligomers having semiconductive or conductive properties when doped with suitable dopants are also known and are described in our copending applications (including U.S. Ser. Nos. 06/773,381, filed Sep. 5, 1985 and 07/212,404, filed Jun. 27, 1988 to Lubowitz, Sheppard and Torre). The linear arms of the oligomers contain conductive linkages, such as Schiff base (--N.dbd.CH--) linkages, between aromatic groups. Sulfone and ether linkages are interspersed in the arms. Each arm is terminated with a mono- or difunctional end cap to allow controlled crosslinking upon heat-induced or chemical-induced curing.